A typical scheme for a wireless communications in the RX section is shown in FIG. 1. The circuit 1 comprises a mixer 10, which is connected to two processing paths 16a, 16b. Both processing paths 16a, 16b each comprise an input processing network 11a, 11b, connected to a programmable gain amplifier (also referred to as PGA) 12a, 12b, which in turn is connected to a low-pass filter 13a, 13b. The low-pass filters 13a, 13b are each connected to a variable gain amplifier 14a, 14b, which in turn is connected to a driver 15a, 15b. 
A differential input signal INP, INN is entered into the mixer 10 and mixed with a local oscillator signal LO_I, LO_Q, resulting in an intermediate frequency signal. An I -component is handled by the first processing path 16a, while a Q-component is handled by the second processing path 16b. The input networks 11a, 11b perform a matching and hand the signal on to the programmable gain amplifiers 12a, 12b, which perform an attenuation. The low-pass filters 13a, 13b perform a low-pass filtering, before the variable gain amplifiers 14a, 14b perform a variable gain amplification. Finally, the drivers 15a, 15b perform an additional fixed gain amplification.
In such systems, the low pass filter 13a, 13b selects the useful signal. This is often used to tune the operation frequency of the filter and the gain of the receiving (RX) Section. The gain is usually programmed with the programmable gain amplifier 12a, 12b. 
The filters 13a, 13b can be used also in other applications. When high linearity is needed, closed loop approaches are preferred over, Gm-C implementations. RC filters fall into three categories, Sallen-Key, BiQuad, Multiple-Feedback (MFB) each one with distinct advantages and disadvantages. Their architecture and design equations are shown in the following.
In FIG. 2, an exemplary BiQuad filter 20 is shown. The filter 20 comprises an input resistor 21, which is connected to a negative input of a first operational amplifier 24. The positive input of the operational amplifier 24 is connected to ground. The output of the first operational amplifier 24 is connected to the negative input of the first operational amplifier 24 by a parallel connection of a resistor 22 and a capacitor 23. The output of the first operational amplifier 24 is moreover connected by a resistor 26 to a negative input of a second operational amplifier 28. The positive input of the second operational amplifier 28 is connected to ground. The negative input of the second operational amplifier is connected to the output of the second operational amplifier 28 by a capacitor 27. The output of the second operational amplifier 28 is moreover connected to an inverter 29, which in turn is connected to a resistor 25, which again is connected to the negative input of the first operational amplifier 24. The resistor 21 forms the input port of the BiQuad filter 20, while the output of the second operational amplifier 28 forms the output of the BiQuad filter. The equations, in particular transfer function, operating frequency and quality factor, of the BiQuad filter are given by:
                    H        ⁡                  (          s          )                    =                                    -                          R              4                                /                      R            1                                    1          +                      s            /                          (                              Q                ⁢                                                                  ⁢                                  ω                  0                                            )                                +                                    (                              s                /                                  ω                  0                                            )                        2                                ,                  ⁢                  ω        o            =              1                                            C              1                        ⁢                          C              2                        ⁢                          R              2                        ⁢                          R              4                                            ,    and        Q    =                                        C            1                                C            2                              ⁢                                    R            3                                                              R                2                            ⁢                              R                4                                                    .            
In FIG. 3, a Sallen-Key filter is shown. The Sallen-Key filter 30 comprises a resistor 31, which is connected to a resistor 32, which again is connected to a positive input of an operational amplifier 36. The positive input of the operational amplifier 36 is moreover connected to ground by a capacitor 34. The connection point between the resistors 31 and 32 is moreover connected by a capacitor 33 to the output of the operational amplifier 36. The output of the operational amplifier 36 is moreover connected by a resistor 37 to the negative input of the operational amplifier. The negative input of the operational amplifier 36 is moreover connected to ground by a resistor 35. The resistor 31 forms the input of the Sallen-Key filter 30, while he output of the operational amplifier 36 forms the output of the Sallen-Key filter 30. The equations of the Sallen-key filter are given as follows:
            R      1        =          α      ⁢                          ⁢      R                  R      2        =    R              C      2        =    C              C      1        =          β      ⁢                          ⁢      C            G    =          1      +                        R          6                /                  R          5                                H      ⁡              (        s        )              =          G              1        +                              sC            2                    ⁡                      (                                          R                2                            +                              R                1                                      )                          +                              R            1                    ⁢                                    C              1                        ⁡                          (                              1                -                G                            )                                      +                              s            2                    ⁢                      C            1                    ⁢                      C            2                    ⁢                      R            2                    ⁢                      R            1                                    Q    =                            α          ⁢                                          ⁢          β                            1        +        α        +                  α          ⁢                                          ⁢                      β            ⁡                          (                              1                -                G                            )                                                      ω      0        =          1              RC        ⁢                              α            ⁢                                                  ⁢            β                              
In FIG. 4, a MFB filter 40 is shown. The MFB filter comprises a resistor 41, which is connected to a resistor 43, which in turn is connected to a negative input of an operational amplifier 46. The connection point between the resistors 41 and 43 is connected to ground through a capacitor 42 and to an output of the operational amplifier 46 through a resistor 44. The negative input of the operational amplifier 46 is moreover connected to the output of the operational amplifier by a capacitor 45. A positive input of the operational amplifier 46 is connected to ground. The first resistor 41 forms the input port of the MFB filter 40, while the output of the operational amplifier 46 forms the output of the MFB filter 40. The equations of the MFB filter are given in the following:
            Q      =                                                  C              2                                      C                              1                ⁢                                                                                                      ⁢                                                            R                2                            ⁢                              R                3                                                                        R              2                        +                                          R                3                            ⁡                              (                                  1                  +                                                            R                      2                                                              R                      1                                                                      )                                                          ,                  ⁢                  ω        o            =              1                                            C              1                        ⁢                          C              2                        ⁢                          R              3                        ⁢                          R              1                                            ,    and              H      ⁡              (        s        )              =                                        -                          R              2                                /                      R            1                                    1          +                      s            /                          (                              Q                ⁢                                                                  ⁢                                  ω                  o                                            )                                +                                    (                              s                /                                  ω                  o                                            )                        2                              .      
From the above shown equations and topologies it can be shown that for a 2nd order section the Sallen-Key filter is not capable of differential mode. Further, it is difficult to implement gain control and frequency tuning.
The BiQuad filter requires two operational amplifiers and therefore consumes a large amount of power.
With the MFB filter is difficult to implement simultaneous gain control and frequency tuning.
This last statement can be understood by referring to earlier elaborations and equations for MFB operating frequency and quality factor, where the operating frequency can be tuned by changing R or C but it is not possible to change the gain of the filer(=ratio R2/R1) without impacting the quality factor Q, i.e. the shape of the filter. Therefore, MFB does NOT support simultaneous frequency and gain control, and if R2/R1 is changed, Q is automatically changed as well.
This disadvantage, i.e. it is not possible to control simultaneously gain and frequency, makes it necessary to use a separate PGA in case a programmable gain control is needed, like in a typical wireless communication system, as shown in the following FIGS. 5 and 6.
In FIG. 5, a filter circuit 50 comprising a programmable gain amplifier 51 and a filter 52 is shown. Therefore, here the programmable gain amplifier 51 is placed before the filter 52. Alternatively, in FIG. 6, a filter circuit 60 comprising a filter 61 and a programmable gain amplifier 62, is shown. Here, the filter 61 is placed before the programmable gain amplifier 62. Both alternatives have the joint disadvantage of the extra programmable gain amplifier 51, 62 drawing extra current and reducing system bandwidth.
In a possible implementation, low-pass filter (also referred to as LPF) and PGA can be implemented with closed loop voltage-mode structures. In this case, a typical wide-bandwidth operational amplifier (OPAMP) is used. Wide bandwidth is given by the single-pole structure, and low output impedance is given by a voltage follower as output stage.